/Engine/Source/Runtime/Experimental/Chaos/Public/Chaos/PBDRigidClustering.h
GetClusterGroupIndexArray The group index is used to automatically bind disjoint clusters. This attribute it set during the creation of cluster to a positive integer value. During UnionClusterGroups (which is called during AdvanceClustering) the positive bodies are joined with a negative pre-existing body, then set negative. Zero entries are ignored within the union.
[TArrayCollectionArray](API\Runtime\Engine\Physics\Experimental\TArrayCollectionArray)< int32 > & GetClusterGroupIndexArray()